Electrical submersible pump assemblies are commonly used in hydrocarbon producing wells to pump well fluid. These assemblies include a rotary pump driven by an electrical motor. A seal section coupled between the pump and motor reduces a pressure differential between well fluid and motor oil or lubricant contained in the motor and part of the seal section. Usually, a string of production tubing supports the submersible pump assembly in the well. A chive shaft extends from the motor through the seal section to the pump. At least one shaft seal seals around the shaft to block the entry of well fluid into the motor and seal section. The well fluid often contains a high percentage of water, which is damaging to internal component so the motor.
Shaft seals are known to leak eventually, thus many submersible pump assemblies fail due to the entry of well fluid into the motor. The failure could be within a few months or years after installation. When a failure occurs, the operator has to retrieve the pump assembly for replacement or repair. Retrieval of a pump assembly suspended on production tubing requires pulling the production tubing, an expensive and time consuming task. Often, the operator will not know whether the failure resulted from encroaching well fluid into the motor or for some other reason.
One solution to reducing the cost of replacing a submersible pump assembly is to suspend two pump assemblies on a Y-tool secured into the production tubing. Each pump assembly has a rotary pump, seal section, and motor. One of the pump assemblies becomes the primary pump assembly, and it is operated initially. The secondary pump assembly will not be operated until the first pump assembly fails. A valve and an intake plug block well fluid from entering the secondary pump until needed, because the well fluid can be corrosive. The secondary pump would be filled with a non corrosive buffer fluid. At startup, the valve opens and the plug is dissolved or discharged to expel the buffer fluid and allow the well fluid to enter the secondary pump.
Also, the secondary pump could be a different type and/or one that produces more efficiently at a lower flow rate than the primary pump. The secondary pump would be employed possibly before the primary pump fails, but when lower well fluid flow into the well justifies using the secondary pump and shutting down the primary pump.
A problem with installing a secondary, non operating pump would occur if the well fluid began leaking into contact with the buffer fluid. By the time the operator wants to start the secondary pump, corrosive well fluid could have entered the secondary pump and damaged the components.